1. Field of the Invention
This invention relates to a white balance correcting device for video cameras, electronic still cameras or other image sensing apparatuses.
2. Description of the Related Art
FIG. 1 is a block diagram illustrating an example of construction of the automatic white balance correcting device for a video camera disclosed in U.S. patent application Ser. No. 395,102 filed on May 31, 1989. The circuitry of the video camera comprises an image sensor 1, a luminance and chrominance signal forming circuit 2, gain control circuits 3 and 4 inserted into signal lines for red color R and blue color B, a color-difference signal forming circuit 5, an encoder 6, gate circuits 7 and 8, an (R-B) signal detecting circuit 9, an averaging circuit 10, a comparison amplifier 11, a limit circuit 12 and a tracking correction circuit 13. The gain control circuits 3 and 4 and the parts beginning with the gate circuits 7 and 8 and terminating at the tracking correction circuit 13 constitute an automatic white balance correcting device 14.
Next, the operation of the camera of this character is described.
A photo-signal entering the image sensor 1 is photoelectrically converted and is taken out as an electrical signal which is supplied to the luminance and chrominance signal forming circuit 2 where a high-frequency component Y.sub.H of a luminance signal, a low-frequency component Y.sub.L of the luminance signal, a red color signal R and a blue color signal B are formed. The red and blue color signals R and B are supplied to the respective gain control circuits 3 and 4, where they are amplified individually in accordance with the characteristics controlled by respective control signals output from the tracking correction circuit 13. After this, they are produced in another form of color signals R' and B' which are then supplied together with the aforesaid low-frequency luminance signal Y.sub.L to the color-difference signal forming circuit 5, where color-difference signals (R-Y.sub.L) and (B-Y.sub.L) are formed. These color-difference signals (R-Y.sub.L) and (B-Y.sub.L) are supplied together with the aforesaid high-frequency luminance signal Y.sub.H to the encoder 6, where a standard television signal is formed and output. Here, the aforesaid color-difference signals (R-Y.sub.L) and (B-Y.sub.L) are also supplied to the automatic white balance correcting device 14.
That is, the aforesaid color-difference signals (R-Y.sub.L) and (B-Y.sub.L) are supplied respectively to the gate circuits 7 and 8, where the unnecessary signals in the blanking period and the abnormal color-difference signals due to the signal collapse at the time of high brightness photography are removed.
The signals output from the gate circuits 7 and 8 are supplied to the (R-B) signal detecting circuit 9. Here, by obtaining a difference between the outputs (R-Y.sub.L)' and (B-Y.sub.L)', an (R-B) signal is detected. In the averaging circuit 10, the (R-B) signal output from the (R-B) signal detecting circuit 9 is averaged, thus being converted to a DC signal. In the comparison amplifier 11, the signal output from the averaging circuit 10 is compared with a reference voltage Vref1. A signal representing the result of this comparison is output to the limit circuit 12. In the limit circuit 12, the signal output from the comparison amplifier 11 is limited to range between a lower limit and an upper limit for color temperatures set at voltages V2r and V3r, so that the white balance is controlled within an actually acceptable color temperature range (for example, 2000.degree. K.-10000.degree. K.). Therefore, the output of the limit circuit 12 lies above the voltage V2r and below the voltage Vr3.
The output of the limit circuit 12 is supplied to the tracking correction circuit 13. In the tracking correction circuit 13, based on the signal output from the limit circuit 12, signals Rcont and Bcont for controlling the gains of the aforesaid gain control circuits 3 and 4 so as to correct the white balance are formed and output, to the gain control circuits 3 and 4 respectively.
Here, an example of the relationship of the signals Rcont and Bcont with the color temperature is described by using FIG. 2 and FIG. 3.
In FIG. 3, a point of white color for a color temperature 6000.degree. K. is denoted by P1, and points of white color for color temperatures 2000.degree. K. and 10000.degree. K. are denoted by P2 and P3 respectively. As the voltages of the signals Rcont and-Bcont at the point P1 have values V1r and V1b, the required voltages of the signal Rcont and Bcont for controlling (correcting) the point P2 to move to the center of the vector diagram of FIG. 3 take values V2r and V2b respectively in FIG. 2. Likewise, in the case of the point P3, they take values V3r and V3b respectively.
Yet another point P4 in FIG. 3 is, however, not brought up to the center P1 of the vector diagram even when the white balance is corrected, because the signals Rcont and Bcont are limited to the voltages V3r and V3b.
Since, in such a manner, the negative feedback loop of the automatic white balance correcting device operates, for color temperatures in the actually acceptable range, the color-difference signal to be supplied to the encoder 6 can take good white balance.
With the above-described arrangement, however, if, an object to be photographed has uneven color temperature distribution, there is a one-sided trait in its distribution, an error is produced in the white balance correction. Hence, there is a problem that the right correcting effect is not obtained. This problem is explained by taking an example of 50% of white color and 50% of blue color. When the gain control circuits 3 and 4 each have a gain factor of one, the white color and the blue color on the vector diagram have their points W0 and B0 in FIG. 4 respectively.
Also, the outputs Rcont and Bcont of the tracking correction circuit 13 are assumed to lie in that relationship with the color temperatures which is shown in FIG. 2.
Since, in this relationship, the operation of the negative feedback loop results in the (R-B) signal becoming zero, the correcting direction of the white balance is parallel to the R-B axis on the vector diagram shown in FIG. 4 or FIG. 7. Therefore, the points to which this negative feedback operation is stabilized are, when the limit circuit 12 does not work, found from B0,B1=B1,a=W0,W1 to be at B1 and W1 (where the line segment B1,a is parallel to the R-B axis and the point "a " lies on a line segment passing through the original point W0 and perpendicular to the R-B axis). But, because the limit circuit 12 operates, it is in reality that they come to points B2 and W2. In other words, the blue color and the white color, though, before subjected to correction, lying at points B0 and W0, change their coordinates to points B2 and W2 respectively after they have been corrected.
This means that when, for example, the blue sky is photographed, the sky image gets a magenta tint as indicated close to the point B2. To avoid this, it may be considered to narrow the limit width of the limit circuit 12. If it is so made, the shade portion of a white object looks in a blue tint. Like this, alternative drawbacks are produced.